The present invention generally relates to a video signal processing apparatus used in an image display apparatus such as a television receiver, a display unit, or the like, and particularly relates to a video signal processing apparatus for converting video signals based on a desired transmission system such as NTSC, PAL, MUSE or any other system so as to conform to a display unit of a desired screen system such as XGA, VGA or the like.
Based on a transmission system of color television signals, conventional television broadcasting systems can be roughly classified into four systems including the NTSC system, the PAL system, the SECAM system and the MUSE system. The MUSE system of them is a band-compression transmission system developed for realizing Hi-Vision broadcasting, and hence designed to be able to manage a much larger information content than any of the other three standard transmission systems. For example, in comparison with the NTSC system adopted as a standard transmission system in Japan with the MUSE system, the information content managed by the latter is about five times as large as the information content managed by the former.
The transmission systems of color television signals described herein are described in detail in the following documents "Television Gijutsu Kyokasho (Television Technology Textbook)" the first volume, edited by Japan Broadcasting Corporation, published by Japan Broadcast Publishing Co., Ltd. (pp. 31 to 91), "MUSE--Hi-Vision Transmission System" edited by Institute of Electronics, Information and Communication Engineers of Japan, Published by Corona Publishing Co., Ltd. (pp. 1 to 8), and so on.
On the other hand, as for graphics in personal computers, video signals (hereinafter referred to as "personal computer video signals") and display units having different standards from one architecture to another are usually adopted. Video signals and display units based on any one standard of VGA mode (640.times.480 dots), SVGA mode (800.times.600 dots) and XGA mode (1024.times.768 dots) are usually adopted in most machine types.
Technology about display units described herein is disclosed in detail in the document "Television Gijutsu Kyokasho (Television Technology Textbook)" the first volume, edited by Japan Broadcasting Corporation, published by Japan Broadcast Publishing Co., Ltd. (pp. 277 to 281), as well as others.
A display unit for displaying images in a personal computer is usually mounted with a video signal processing apparatus for modulating the signal system of video signals supplied to the personal computer so as to conform to the standard of the display unit.
The basic structure of a conventional video signal processing apparatus will be described with reference to FIG. 9. As for the conventional technology described herein, it is assumed by way of example that a picture image represented by personal computer video signals of the VGA mode is displayed on the screen of a display unit (such as a liquid-crystal display unit) of the XGA mode. In this example, it is assumed that personal computer video signals having general standards (frame frequency fv is 59.94 Hz, horizontal frequency fh is 31.47 Hz, and resolution including a blanking period is 800 dots.times.525 dots) are used.
A conventional video signal processing apparatus has an input terminal 1 supplied with personal computer video signals from a video signal output apparatus, an A/D (analog-to-digital) converter 2 for digitally converting, with a predetermined sampling frequency (800.times.fh Hz), the personal computer video signals supplied through the input terminal 1, and a picture image magnifying circuit 901 for performing well-known picture element interpolation upon the personal computer video signals digitally converted by the A/D converter 2 so as to magnify a picture image represented by the personal computer video signals horizontally and vertically respectively at a predetermined ratio (1.6 times).
The personal computer video signals supplied from the picture image magnifying circuit 901 are supplied to a liquid-crystal display unit 8 as output signals of this video signal processing apparatus. As a result, a picture image represented by the personal computer video signals of the VGA mode are displayed properly on the screen of the liquid-crystal display unit 8 of the XGA mode.
As personal computers have come into wide use recently, image display units are being requested to be able to display with high quality both picture images from a personal computer and picture images of television signals. However, the above-mentioned video signal processing apparatus has a problem, in that it is impossible to deal with video signals (such as color television signals or the like) other than personal computer video signals properly. For example, when double-speed television signals of the NTSC system (field frequency fv is 59.94 Hz, horizontal frequency fh is 31.5 kHz, and resolution including a blanking period is 910.times.525 dots) or Hi-Vision signals of the MUSE system (field frequency fv is 60 Hz, horizontal frequency fh is 33.75 kHz, and resolution including a blanking period is 1920.times.562.5 dots) are inputted, in an A/D converter 2, the video signals are digitally converted by the same sampling frequency (800.times.fh Hz) as that in the case where personal computer video signals are inputted. Accordingly, information included in the original color television signals is thinned out. Therefore, the resolution decreases to 800/910 when the double-speed television signals of the NTSC system are inputted, and the resolution decreases to 800/1920 when the Hi-Vision signals of the MUSE system are inputted.
Further, a picture image represented by the Hi-Vision signals of the MUSE system is reproduced as an unnaturally distorted image. That is, the aspect ratio of the screen of a standard display unit used for displaying picture images in a personal computer is 4:3 while the aspect ratio of the screen preferred for Hi-Vision signals of the MUSE system is 16:9. Therefore, the picture image is distorted by the difference in the aspect ratio between the two.